Supernormal muscle fiber conduction velocity during intermittent isometric exercise in human muscle.

Muscle fiber conduction velocity (MFCV) and surface electromyographic parameters were studied in the brachial biceps muscle of healthy males during voluntary intermittent isometric contractions at 50% of maximum force. Recovery in the following 15 min was then observed. The measurements were performed during duty cycles of 33, 25, and 20%. The main finding was a supernormal MFCV during the exercise phase when the duty cycle was 25 and 20%. The level continued to increase during the recovery phase. During the exercise phase when the duty cycle was 33%, the MFCV decreased slightly (suggesting that the local anaerobic threshold had been surpassed) but increased during recovery to supernormal values. The ratio of median frequency to MFCV was constant during all experiments, indicating that the changes in median frequency reflect those in MFCV. We suggest that the supernormal MFCV was due to a combination of altered membrane properties, muscle fiber swelling, and temperature increase and hypothesize that the changes of electrical properties formed part of an adaptive mechanism of the muscle fiber membrane during exercise. In that respect, the increase of the MFCV could be a component of the well-known warm-up effect.